Marine engines used to power watercraft, such as a boat, are susceptible to being damaged through introduction of water. Water injection can occur in a marine engine in several different manners. Although four such manners will be discussed it is to be understood that other types are possible. The first means is through wave action. Here, a surge of water enters the exhaust track and proceeds into the engine. The surge of water is produced from an external source such as the wake of a passing boat, inclement weather or turning of the watercraft. The second way in which water can be introduced into the marine engine is through by-products of the combustion process. Marine engines have exhaust manifolds that operate at a lower temperature than land based engines due to the fact the engine must be safely designed in order to be enclosed in a watercraft. The aforementioned means of water injection can be controlled through proper design elements in the marine engine manifold, calibration of the marine engine, and through proper operation and maintenance of the watercraft.
A third way of introducing water into the marine engine is by way of condensation. Condensation may occur through having dissimilar cooling rates of various components subsequent to shutting down the engine. Temperature differences between daytime and nighttime may also cause condensation to form in the engine exhaust system. Further, condensation may also result from having too low of an operating temperature of the exhaust system. Condensation can be controlled through design of all of the components of the exhaust system. In this manner, an elbow of the manifold, exhaust angle and exhaust hoses can be designed and selected to minimize or eliminate the occurrence of condensation in the system.
The fourth way through which water may be introduced into a marine engine is through reversion. This manner of water injection is the hardest to control. Reversion is the backwards flow of exhaust gases during the time period in which both intake and exhaust valves are simultaneously open. Pulses in the exhaust system cause water to work its way backward into the exhaust manifold. Reversion primarily occurs when the engine runs at idle speed or slightly above idle speed. Reversion can be controlled through manifold design and operating temperature. Further, engine calibration and certain camshaft designs can be used to reduce reversion in marine engine exhaust systems.
Water injected into such an engine typically damages an exhaust valve thus preventing the cylinder with the damaged exhaust valve from correctly sealing. This damaged cylinder then causes water to be pulled into the engine through the corrupted exhaust valve. This water is redistributed to the rest of the engine and causes its ultimate failure. The control of water injection is a primary objective of watercraft and engine manufacturers and is especially challenging considering the environment into which the watercraft is deployed.
One known type of marine engine exhaust system design that seeks to minimize reversion employs connected conduits from a pair of manifolds on either side of the engine. Gas pulses from each conduit are combined and are subsequently injected with cooling water. This type of system seeks to combine pulses from either side of the engine so that double the number of pulses are present during engine idle. Unfortunately, the exhaust gases in such a system are extremely hot because water is not added until some point after the gases combine. This arrangement increases backpressure on the engine.
Over the years, marine engine exhaust systems have been proposed to minimize water reversion as well as the other three conditions capable of causing water ingestion. Although current systems have achieved some degree of success, no classically designed system exists that is capable of completely eliminating all means of water entry into a marine engine. As such, there remains room for variation and improvement within the art.